Mass spectrometry allows the determination of the mass-to-charge ratio (m/z) of ions of sample molecules. This technique involves ionizing the sample molecule or molecules and then analyzing the ions in an analyzer and detecting the analyzed ions. Various mass spectrometers are known.
Tandem mass spectrometry is an exemplary use of a mass spectrometer to gain structural information about the sample molecule or molecules. This common type of spectrometry includes generating sample ions, subjecting the ions to a first stage of mass analysis, reacting one or more of the ions (referred to as parent ions) analyzed in the first stage of mass spectrometry, and then analyzing the ions that are products of the reaction (products ions) with the second stage of mass analysis and detecting the analyzed ions. The ion trap can be utilized for selecting parent ions of a desired mass-to-charge ratio (m/z) for analysis. The parent ions are then dissociated into product ions, which may be analyzed by the same mass analyzer to determine the mass-to-charge ratios of the products ions and obtain a mass spectrum of the products ions.
Recently, the desire for improved ion trap performance has led to further exploration of higher order field components. Most notably has been the introduction of small amounts of octapole and hexapole higher order field components to the quadrupole ion trap. Because of the inherent asymmetry, hexapole fields improve ejection efficiency thus enhancing the sensitivity of a quadrupole ion trap. Increasing the octapole electric field component in a quadrupole ion trap has been used to correct for the electric field deformation caused by the opening in the endcap electrodes, which enhances the mass accuracy and resolution of the quadrupole ion trap. An additional advantage of octapole fields is an improvement in the efficiency of tandem mass spectrometry due to the cross terms (r2z2) in the ions' motion within octapole fields. Although quadrupole ion traps with higher order fields have enhanced analytical performance, there still remains a desire to further improve performance with regard to sensitivity, ion detection methods, ion ejection and MS/MS efficiencies.
A typical quadrupole ion trap includes a ring electrode and two endcap electrodes each having an opening for passage of ions into or out of the trapping volume. In order to trap charged particles, the ion trap uses a dynamic voltage applied to the ring electrode and/or the endcap electrodes to confine charged particles within the trapping volume. The quadrupole ion trap is a three dimensional analog to a linear (two-dimensional) quadrupole mass filter. Both are used successfully as mass spectrometers. Two dimensional quadrupoles are also used as ion guides, to efficiently transport ions in various types of mass spectrometers. Higher order linear multipoles, such as hexapoles and octapoles, have also been used as ion guides, but never as mass spectrometers. Although quadrupole ion traps have reasonable analytical performance, there still remains a desire to further improve performance with regard to sensitivity, ion detection methods, ion ejection and MS/MS efficiencies.
Accordingly, in light of desired improvements associated with ion trapping and ejection, there exists a need for improved ion trap mass spectrometers and related methods.